When optical lines are accommodated by using wavelength division multiplexing (WDM), a network management system allocates wavelengths to the respective optical lines. A process for allocating wavelengths to optical lines to be allocated may be referred to as wavelength allocation design. Note that an “optical line” in the description below corresponds to an optical path (or a wavelength path) that is established between two nodes.
In the wavelength allocation design, wavelengths are allocated to respective optical lines that are established on each optical fiber in such a way that the wavelengths of the respective optical lines are different from each other. In addition, in order to reduce the cost of a network, the same wavelength is used on a route from a start point node to an end point node of each of the optical lines in many cases.
In recent years, an operation scheme in which an optical line of a needed bandwidth is established as needed has been widely used according to, for example, software defined networking (SDN). Namely, an optical line may be frequently added or deleted. Therefore, even in a case in which wavelengths are allocated to respective optical lines in such a way that wavelength usage efficiency is high at a particular point in time, when an optical line is added or deleted afterward, the wavelength usage efficiency may decrease. Accordingly, an operation is requested in which the wavelength usage efficiency is monitored and wavelength allocation is changed when the wavelength usage efficiency decreases. An action to change wavelength allocation during operation may be referred to as wavelength reallocation or defragmentation (or simply referred to as “defrag”).
In the example illustrated in FIG. 1A, in a WDM optical network including nodes A-F, thirteen optical lines are established by using wavelength slots 1-11. However, wavelength usage efficiency is not high in this wavelength allocation. As an example, wavelength slot 1 is used by a link between node E and node F, but is not used by respective links between node A and node E. In addition, wavelength slot 2 is used by respective links between node A and node D, but is not used by respective links between node D and node F. Note that a wavelength slot corresponds to a minimum unit of wavelength resources allocated to an optical line.
FIG. 1B illustrates an example of a result of performing wavelength reallocation on the WDM optical network illustrated in FIG. 1A. In this example, wavelength slots 8-11 are not allocated to any optical lines. Namely, wavelength slots 8-11 can be allocated to new optical lines in arbitrary links. As described above, wavelength reallocation enables the wavelength usage efficiency of a WDM optical network to be improved.
As a related technology, a wavelength path reallocation method for designing wavelength paths in such a way that a used frequency area becomes smaller than before reallocation has been proposed (for example, US Patent Publication No. 2013/0195460). A method for reoptimizing a network while reducing the number of cancellations of demands allocated to slots has been proposed (for example, Japanese Laid-open Patent Publication No. 2014-229938). Further, a method for performing wavelength reallocation while suppressing an influence on optical lines has been proposed (for example, Y. Takita et al., Wavelength Defragmentation with Minimum Optical Path Disruptions for Seamless Service Migration, OFC2016, M2J.3).
As described above, wavelength reallocation enables the usage efficiency of wavelength resources of a WDM optical network to be improved. However, it is not easy to determine an optimum procedure for changing wavelength slots according to the wavelength reallocation. As an example, it is not easy to determine a procedure for changing wavelength slots allocated to respective optical lines without disconnecting any optical lines.
According to the above paper, Y. Takita et al., a procedure for changing wavelength slots allocated to respective optical lines while minimizing the disconnection of optical lines can be determined. However, when the number of nodes in an optical network is large and its topology is complicated, it takes a long time to determine an optimum wavelength allocation and a procedure for changing wavelength slots. Therefore, in a conventional technology, it is difficult to dynamically change wavelength allocation of a large-scale WDM optical network.